Broadcast cell and coordinate shape in from_cell_list

These have to be consistent across all processes.
Fail parallel unit circle test more gracefully

When running in parallel we can't use pytest's importorskip, instead try
importing gmshpy and mark the test as skipped if we fail.
Merge pull request #220 from firedrakeproject/fix/meshing

- Broadcast correct shape of cells and coords in from_cell_list.
- Skip parallel gmsh tests more gracefully if gmshpy is not installed.